The temperature: 10 below zero. The location: the middle of frozen Waite Lake in the Northwest Territories, 1000 miles north of the U.S. border. I'm with six Canadian ice-road experts on the shoulder of a highway that curves from the powder-frosted shoreline forest, across the lake and into the distance. In the pale light of winter, even the sun seems frozen.

Fifty yards away, a tanker truck hauling 40 tons of fuel oil inches forward, its huge diesel rumbling. I'm startled to hear the ice beneath our feet make a sound like shattering window glass, but no one else seems to notice. Apparently, ice that's 3 ft. thick behaves this way when you drive a massive truck over it.

But something else I notice is definitely not normal. A few yards from the road, Waite Lake's smooth surface rears up in jagged shards; beyond is a pool of black water. The formation is called a blowout, a slow-motion upheaval of ice that produces what looks like a bomb crater. As the tanker eases past, the water rises, laps the blowout's shattered margins, then subsides. The experts watch intently in silence. When it's your job to maintain a road made of ice, the last thing you want to see is water.

Here in the Northwest Territories, the terrain is all but impassable for much of the year, a vast wilderness of lakes, boreal forest and spongy tundra. Nearly twice the size of Texas, the Northwest Territories are home to only 42,000 souls and just 570 miles of paved road. If you want to get almost anywhere, you have to fly.

Then, in early November, winter comes. As temperatures plummet, the lakes freeze over and the marshes turn rock solid. Once the ice is a foot thick -- usually by late December -- snowplows fan out into the hinterlands, blazing routes to native villages and mining camps by clearing insulating snow off the ice to speed the thickening process.

When it comes to epic northern engineering, nothing tops the Tibbitt to Contwoyto Winter Road, a superhighway of ice that extends 370 miles from north of Yellowknife into the neighboring territory of Nunavut. To build it, 140 workers from the Nuna Logistics construction firm struggle through 20-hour nights and windchills that dip to 70 below. By the end of January, they have completed the longest heavy-haul ice road in the world, as wide as an eight-lane highway. When the ice thickens to 40-plus in. -- typically, in late February -- it is capable of supporting 70-ton eight-axle Super B Train articulated trucks.

The road services mines that tap into rich deposits of diamond-bearing kimberlite. Since the first samples were found here in 1991, Canada has gone from marketing no diamonds to being the world's third largest producer by value (after Botswana and Russia). Last year, two mines in the territories produced more than 12 million carats, worth $1.5 billion (U.S.). This year another mine will open, at Snap Lake, halfway up the Tibbitt to Contwoyto road. To operate the mines, 300,000 tons of fuel, explosives, steel and concrete must be hauled in over the ice each year.

If you want to learn about Canadian ice roads, sooner or later you have to talk to John Zigarlick. In the early '80s, as president and CEO of Echo Bay Mines, he oversaw the construction of a gold mine 250 miles north of Yellowknife, and the ice road to reach it. After Zigarlick retired in 1998, he waited all of about two weeks before founding a new company, Nuna Logistics, and convinced Echo Bay to let him manage the road.

In those days, before diamond mining, a typical winter saw 700 to 1000 truckloads run north on the ice road, mostly to the gold mine. Fast forward seven years: With the diamond business exploding, demand for haulage has increased tenfold. But there's one problem facing Zigarlick's road -- a little thing called global warming.

"This is the worst bloody year ever," Zigarlick says, still eyeing the Waite Lake blowout. An affable, soft-spoken man of 69, he's not prone to exaggeration. According to scientists, the winter of 2005-06 is the warmest since recordkeeping began in 1850. In December 2005, when the mercury should have read 10 below zero, temperatures hovered around freezing. The ice didn't thicken enough to bear traffic until Feb. 5, 2006. Now it's late March, and it still hasn't reached the 40 in. needed to support fully loaded trucks. This season is no fluke: With the exception of two springs, 2002 and 2004, seasonal temperatures in the Canadian Arctic have remained above normal for the past eight years.

Earlier today, when Zigarlick and I left the gravel highway that ends about 40 miles northeast of Yellowknife and headed out onto the ice road, we frequently encountered long, inch-wide cracks. An hour in, as we crossed a small pond, we passed a cordoned-off area where a plow had fallen through the ice. The most worrisome stretch of road is here on Waite Lake. For reasons no one quite understands, the ice in one 20-ft. section has failed to thicken properly, forcing road managers to come up with creative solutions. Out in the middle of the lake, engineers have laid down a "rig mat," a lattice of steel and wooden beams, and frozen it into the ice to bridge the weak spot. It should hold -- for now. But the season doesn't end until early April, and thousands of truckloads are still waiting to be dispatched from Yellowknife.

Zigarlick and I continue north in his Ford F-350 Super Duty pickup. The trees become shorter and sparser until they disappear altogether. Where the route rises onto portages between lakes, Nuna's crews pack down ice and snow over a gravel bed. But 85 percent of the road lies over lakes. Some are so long they take hours to cross; one is nicknamed Two Movie Lake because that's how many DVDs the truck drivers can watch as they crawl along.

The wide, straight road -- white against the white landscape -- yields a monotony that never quite numbs the reality of danger. Many seasoned veterans forgo seatbelts; if the surface gives way, a trucker will have mere seconds to jump clear. The most recent fatality in the Northwest Territories was a 23-year-old who was plowing an ice road near Yellowknife; he drowned when his truck broke through. In 2000, a Nuna worker's plow plunged through the ice on the Tibbitt to Contwoyto road, and though fellow workers pulled him clear, the shock of frigid water and freezing air triggered a fatal heart attack.

Near twilight, as we cross Lac de Gras, tendrils of wind-borne snow twist over the turquoise ice like cold smoke. A light snow begins to drift down; the sun is a fuzzy orange ball in a sky as featureless as the landscape. Then comes nightfall. It's 22 below and winds gust to 46 mph, whipping the drifting snow into an opaque froth. Zigarlick loves this kind of weather: "If it holds for two weeks, we'll be fine."

The basic mechanics of ice roads have been known for decades. As a laden truck moves over ice, it creates a shallow depression all around it -- a sort of bowl in the ice, several inches deep and many yards across. The greater the speed, the deeper the depression. Above a critical velocity that varies with local conditions, a truck can damage the roadbed so severely that the next vehicle to come along will break through the ice. For this reason, the top speed on the Tibbitt to Contwoyto route is usually about 22 mph. In some stretches, as on Waite Lake, the maximum is just a few miles per hour.

As the concavity moves along beneath the truck, the displaced water forms pressure waves under the surface. When these waves reach the lakeshore, they bounce back, and the resulting interference with incoming waves can push the ice up and create a blowout. Whenever possible, engineers build ice roads at an angle to nearby shorelines to prevent blowouts under the roadbed itself.

Every day, a crew drives along the road, generating a profile of ice thickness with ground-penetrating radar. They drag an antenna, which sends 400-MHz radio waves down into the lake. A computer calculates the ice thickness based on the time it takes the signal to return from the ice-water interface. For even greater accuracy in calculating how much weight the road can bear, the data is calibrated against measurements taken from boreholes that workers drill with power augers.

One of the men driving the road with us today is Sam Proskin, a geotechnical engineer whose firm has been hired to help better understand what's going on beneath the ice. Proskin is trying to develop multidimensional maps of problem spots by using more sophisticated radar. This, he hopes, will help solve a vexing mystery: how water flow, sandbars and other subsurface features affect the underside of the ice. Such an understanding could help engineers re-route the road to minimize problems like the one here on Waite Lake.

"The physical properties of ice are well understood under laboratory conditions," Proskin says, "but when you extrapolate it into the scale of kilometers of road, it's a little more difficult to understand. It changes, it flows, it gets brittle. The ice is like a living thing."

For the roadbuilders, that means working with phenomena for which there is limited scientific understanding. For example, driving at a speed and weight that is appropriate for an ice sheet makes the ice stronger. Truckers say that they're "driving down the frost," but there's no evidence that such a physical process takes place: It's just another mystery of the ice. "Ice is an engineering challenge," Proskin says. "It's a solid near its melting point. If you heated steel to near its melting point and put loads on it, it would act strangely, too."

When we arrive at the Lac de Gras camp, the road's northernmost outpost, we hear the news: This afternoon on Great Slave Lake, 200 miles south, a snowplow fell through the ice. The driver survived, but his mishap is a sobering reminder of the danger.

Like the two other roadside camps, Lac de Gras is warm and carpeted. It has bunks for 49 Nuna staffers, a cafeteria, and lounges with widescreen satellite TV. Occasionally, truckers are invited in to use the facilities, but they sleep in their rigs, which are kept running at around 1200 rpm so the engines don't freeze. The prefabricated structure has the cozily antiseptic feel of a space station. With few windows, there is little sense of where you are in the universe. Everyone walks around in stocking feet.

But reality is just outside the door, waiting for us. The next morning dawns a whitish gray blur: A whiteout has descended on the northern half of the route. At 5:30 am, Zigarlick reluctantly closes the road to trucking for 24 hours. But several trucks that headed south from Tahera diamond mine at 2:50 am remain unaccounted for. I hop in another pickup with chief ice profiler Albert Brandl. The visibility is 30 ft. "This is nothing," Brandl says. "I've had to get out and feel the road to make sure I was still on it."

We drive north, crossing lake after lake. At last we come to a pair of fuel tankers on a portage, the lead truck stuck in a snowbank. When a grader shows up, Brandl hooks a tow rope from the grader to the rear of the lead truck. The grader strains in low gear, but the truck doesn't budge. The grader backs up and gets a running start. The truck lurches a few feet backward. Two more tugs, and it's clear. Smiling, Brandl gets back in the cab. In the 5 minutes he's been outside, his cheeks have turned white with incipient frostbite.

That evening the storm abates, and the following morning Zigarlick drives me down to Yellowknife for my flight home. When I call him a week later, weariness and resignation dampen his voice. "We had to shut the road down the day before yesterday," he says. "For a couple of days, the temperature was well above freezing. Water was seeping through the fractures."

For the first time, the ice road has failed to deliver a full season's worth of supplies -- only 6800 of 9000 loads made it through. The stranded freight has to be flown in, at six to eight times the cost. All told, the early closure costs the diamond mines tens of millions of dollars.

By its very nature, weather is a crapshoot, and future winters may be frigid. But it's now clear that long months of reliable ice can no longer be taken for granted. As Zigarlick knows better than anyone, making a highway out of ice can never be risk-free. "The more you understand the ice," he says, "the more you fear it."

Loaded with diesel for diamond mines, tankers on the Tibbitt to Contwoyto Winter Road wait their turn to cross a frozen lake in Canada's Northwest Territories. Last winter, for the first time, the ice didn't thicken to the consistent 40 in. required to support the heaviest rigs.